1. Field of the Invention
The present invention relates to high-density information storage units (e.g. optical disc units) of the type adapted for optical writing and/or reading (hereinafter write/read) with tightly focused, high intensity light beams, and more specifically to improved protective cover structure in such units.
Brief Description of Prior Art
Because of their great potential for high density information storage, there is active developmental interest in optical write/read system such as optical disc systems. Such systems typically form micron size information bits (e.g. light modulating marks) along recorded tracks on a surface of the disc. One common way to record (write) such marks is by scanning the optical disc with a tightly focused laser beam which is turned on and off according to an encoded signal that contains the information to be recorded. On playback (read), the tracks are illuminated by a tightly focused reading light beam and variations in the light passing from marked and unmarked track portions to a photodetector are sensed to reproduce the encoded signal. Although the disc configuration is currently the most popular format for use in such optical write/read systems, other formats are useful. This specification describes such optical storage media as optical discs with the understanding that there are other formats that will have equivalent utility in accord with the present invention. Also, the term "light", when used herein in its general sense, is intended to include electromagnetic radiation outside the visible spectrum, e.g. U.V. and infrared radiation.
One approach that is commonly used in recording information marks is for the optical disc to comprise a record layer (e.g. solvent coated, sputtered or vapor or vacuum deposited) which includes a material (e.g. a dye or pigment) that is absorptive to light of the writing wavelength. The high intensity writing light is absorbed by the record layer and forms marks, e.g., by a thermal deformation mechanism (such as ablation), by changing the light absorption characteristic of the record layer (such as dye bleaching) or by changing other light modulating properties (such as polarization characteristics).
Reading of the recorded marks can be with the same light wavelength or with a different light wavelength and can utilize intensity modulation of the read light beam (e.g. caused by absorption differences of marked and unmarked track portions), phase or diffraction modulation (e.g. caused by pit-like markings) or combinations of such light modulations or other modulations.
Due to the minute size of the recorded marks it is very important to protect the record surface of the optical discs from scratching and from dirt or dust particles. One general technique for accomplishing this is to provide a light transparent cover structure (e.g. an overcoat layer or a cover sheet) which has an exterior surface sufficiently spaced from the record surface so that scratches, dirt and dust thereon do not interfere with writing or reading of the underlying record surface. There are several known optical disc formats incorporating such covers.
It has been noted that with respect to certain record layer compositions that a decrease in writing (and sometimes reading) sensitivity occurs over time. Such decreases can have various causes, e.g., changes in the record layer composition or structure caused by temperature and humidity. However, it is believed that in some instances a contributing cause for such sensitivity loss is light-induced degradation in record layer absorption, e.g. bleaching or fading.